1. Field of the Invention
The present invention relates to a drive circuit that selectively cyclically drives a row of LEDs (light emitting diodes) used in an electrophotographic printer, a row of heat-generating resistors used in a thermal printer, and a row of display elements used in a display. The invention also relates to an LED head incorporating the drive circuit.
2. Description of the Related Art
In a conventional electrophotographic printer, the surface of a photoconductive drum is charged to a high voltage. An optical writing means such as a light emitting diode (referred to as LED hereinafter) head illuminates the charged surface of the photoconductive drum to form an electrostatic latent image thereon. The electrostatic latent image is then developed with toner into a toner image. The toner image is transferred to a print medium such as paper. The print medium is then advanced to a fixing unit where the toner image on the print medium is fused into a permanent print.
The LED head includes a large number of LED array chips, each of which includes a row of a plurality of LEDs. The LED array chips are aligned in a direction parallel to the rotational axis of the photoconductive drum such that the LEDs of the LED arrays lie on a straight line and the light dots emitted from the LEDs are focused on the surface of the photoconductive drum.
FIG. 17 illustrates a general layout of the conventional driver IC mounted on a printed wiring board and surrounding electrodes formed on the board (e.g., Japanese Patent Preliminary Publication (KOKAI) No. 6-297765/1994). A plurality of input electrodes 31 are arranged on one of opposed long sides of a rectangular driver IC. Two rows of LED drive electrodes 32 are arranged parallel to each other on the other of the opposed long sides such that electrodes in one row are staggered with those in the other.
The driver IC incorporates the row 31 of input electrodes, a row 33 of shift registers, a row 34 of latch circuits, a row 35 of pre-buffers including AND circuits and inverters, and a row 37 of drive transistors in the form of P-channel MOS transistors, all of which are aligned in this order from the input electrode side to the LED drive electrode side. The rows are parallel to one another and extend in the longitudinal direction of the driver IC.
The shift registers, latches, pre-buffers, and the drive transistors are arranged at substantially the same intervals as the LED drive electrodes 32 in the longitudinal direction of the driver IC 11. All of these circuit components receive their control signals from the input electrode side via wires, not shown.
The LED supply voltage electrode 36 is formed of aluminum and in the shape of a belt having a width W. The LED supply voltage electrode 36 lies between the row of the pre-buffers 35 and the row of the drive transistors 37 and extends in a direction parallel to these rows. The LED supply voltage electrode 36 has a plurality of electrode pads 38a-38c (e.g., three pads shown in FIG. 17) mounted thereon through which the LED supply voltage VDDH is supplied for driving the LEDs.
Each driver IC 11 drives a total of 192 LEDs through electrode pads DO1-DO192. The LED drive supply VDDH is supplied through the electrode pads 38a, 38b, and 38c provided at locations that correspond to the electrode pads DO32, DO96, and DO160, respectively.
FIG. 18 is a cross-sectional view illustrating a printed wiring board on which the aforementioned the row 31 of input electrodes, LED drive electrodes 32, and LED supply voltage electrode 36 are formed. The driver IC is not shown in FIG. 18. The LED drive electrodes 32 are wire-bonded to the LED array, not shown, via the electrode pads DO-DO192 in the form of an aluminum pattern formed on the printed wiring board.
The electrode pads 38a-38c are formed on the LED supply voltage electrode 36. The input electrodes 31 are in the form of an aluminum electrode and electrode pads 38a-38c of the driver IC are connected through bonding wire 40 to electrodes, not shown, on the printed wiring board.
FIG. 19 illustrates a pertinent portion of an equivalent circuit of the driver IC of FIG. 17 and an LED array chip driven by the driver IC.
Referring to FIG. 19, one driver IC drives 192 LEDs fabricated on one LED array chip. Resistors R201-203 represent equivalent resistance values of the bonding wires 40 (FIG. 18) through which the individual LEDs receive the supply voltage. Nodes S1-S192 indicate the locations on the LED supply voltage electrode 36 to which the sources of the drive transistors M1-M192 are connected. Resistors R1-R19l represent the resistance values between adjacent nodes.
The electrode pads 38a, 38b, and 38c on the LED supply voltage electrode 36 of FIG. 17 are arranged near the electrode pads DO32, DO96, and DO160, respectively. Therefore, the resistors R201, R202, and R203 are connected to the sources of drive transistors M32, M96, and M160, respectively, i.e., the nodes S32, S96, and S160.
The drains of the drive transistors M1-M192 are connected to the anodes of the LEDs D1-D192. The gates of the drive transistors M1-M192 are connected to a later described controller, for example, shown in FIG. 20, which generates a predetermined gate-to-source voltage Vcont that sets the value of drive current Io flowing through each of the LEDs (FIG. 20 shows only Io flowing through D1).
The P-channel MOS transistor M0 is a reference transistor that generates a reference current Iref. The reference transistor M0 is aligned with the row of drive transistors M1-M192. The reference transistor M0 is located adjacent the drive transistor M1 and is depicted in solid black (FIG. 17). The resistors R0-R191 are equivalent resistance values between adjacent nodes of the LED supply voltage electrode 36 formed in the shape of a belt having a width W.
The drive currents Io that flow through individual LEDs are determined in reference to the reference current Iref that flows through the reference transistor M0 located at an end of the row of drive transistors M1-M192 (FIG. 17).
FIG. 20 illustrates a part of the control voltage generating circuit that provides the Vcont for driving the drive transistors of FIG. 19.
Each driver IC has a control voltage-generating circuit 20 and 192 pre-buffers G1-G192. FIG. 20 shows the pre-buffer G1 and an associated circuit that includes a latch circuit LT1, a P-channel MOS type drive transistor M1, and a light emitting diode D1. D1 is one of the LEDs D1-D192 of FIG. 19. The drive transistor M1 is one of the transistors M1-M192 of FIG. 19. The pre-buffer G1 includes an AND circuit AD1, a P-channel MOS transistor TP1, and an N-channel MOS transistor TN1.
An OP amplifier 21 generates an output voltage Vcont. A P-channel MOS transistor M0 has the same gate length as 192 drive transistors M1-M192. An inverting input of the OP amplifier 21 receives a reference voltage Vref from an external circuit.
The OP amplifier 21, P-channel MOS transistor M0, and a resistor Rref form a feedback control circuit. The current that flows through the resistor Rref, i.e., through the P-channel MOS transistor M0 is determined by the reference voltage Vref and the resistor Rref if the supply voltage of the P-channel MOS transistor: M0 is constant.
FIG. 21 illustrates the aforementioned LED array chips; CHP1-CHP26 aligned along the surface of the photoconductive drum in a direction parallel to the rotational axis of the photoconductive drum, and drive currents Io flowing through the LEDs contained in each of the LED array chips CHP1-CHP26. The LED array chips CHP1-CHP26 are driven by corresponding driver ICs DRV1-DRV26.
Each of the LED array chips CHP1-CHP26 includes 192 LEDs fabricated therein. Each LED is connected by wire bonding to a corresponding electrode pad DO of the LED drive electrode 32 of a corresponding driver IC.
The driver ICs DRV1-DRV26 are connected in cascade so that print data received from an external circuit is serially transferred therethrough. Each of the driver ICs DRV1-DRV26 is capable of driving 192 LEDs.
It is desirable that all of the driver ICs DRV1-DRV26 supply substantially the same drive current Io to all of the LEDs of the corresponding LED array chips. However, the drive current Io vary from LED to LED due to problems encountered during the manufacturing process of semiconductor device. The variation of drive current Io causes variation of the intensity of light emitted from the LEDs. Differences in the intensity of light result in variation of exposure effect when the LED head illuminates the photoconductive drum. This causes the variation of dot-size.
When characters are printed, variation of dot size has negligible effect. However, when an image such as a photograph is printed, the variation of dot size causes the variation of print density, degrading print quality. In order to minimize the variation of dot size, the driver ICs are screened, and selected driver ICs are used so that the drive currents in each LED array chip are within a predetermined range xcex94I.
As is clear from FIG. 21, adjacent LEDs have very small differences in drive current. Drive currents supplied to the LEDs in an LED array chip from the same driver IC tend to monotonically increase or decrease along the row of the LEDs.
For this reason, the current that flows through the electrode pad DO1 (i.e., drive transistor M1) serves as a reference with respect to which the drive currents Io of the individual LEDs are set. The control voltage generating circuit 20 in the respective driver IC controls the drive current Io to be constant. Since selected driver ICs are used, the drive current varies within the predetermined range xcex94I.
Referring to FIG. 21, it is to be noted that only the leftmost D1 of each LED array chip is supplied with a drive current of I1 but other LEDs in the same LED array chip are supplied with drive currents different from I1.
Thus, the conventional driver ICs suffer from the problem that when a plurality of drive ICs are used to drive a plurality of LED array chips, one particular LED in each LED array chip is supplied with the same current as the corresponding LED in the other LED array chips, but no attempt is made to equalize the drive currents supplied to the other LEDs in the LED array chips. Because the drive currents supplied to the LEDs either monotonically increase or monotonically decrease along the row of the LEDs, the drive current may vary within the range 2xcex94I across the entire row of the LED array chips.
The present invention was made in view of the aforementioned drawbacks.
A drive circuit drives a plurality of LEDs in an LED array chip. The drive circuit includes a row of drive devices (e.g., transistors), a control circuit, and a supply voltage electrode. The control circuit has a reference current generating device (e.g., transistor) and generates a control voltage that causes a predetermined reference current to flow through the reference current generating device. The control voltage is also supplied to the drive devices to cause drive currents to flow through the corresponding elements (i.e., LEDs) in reference to the reference current. The supply voltage electrode extends along the row of drive devices, and supplies a supply voltage to the drive devices such that each of the drive devices receives the supply voltage from a nearest location on the supply voltage electrode. The reference current generating device receives its supply voltage from a substantially mid point of the supply voltage electrode.
Another drive circuit drives a plurality of LEDs in an LED array chip. The drive circuit includes a row of drive devices (e.g., transistors) that supply drive currents to corresponding elements (i.e., LEDs), a control circuit having a plurality of reference current generating devices (e.g., transistors) that cooperate to produce a predetermined reference current, and a supply voltage electrode that extends along the row of drive devices. The control circuit generates a control voltage that controls the plurality of reference current generating devices to produce the predetermined reference current. The control voltage is also supplied to the drive devices to cause the drive currents to flow through the corresponding elements in reference to the reference current. The supply voltage electrode supplies a supply voltage to the drive devices such that each of the drive devices receives its supply voltage from a nearest location on the supply voltage electrode. The plurality of reference current generating devices are aligned with the drive devices such that the reference current generating devices are mirror images of one another with respect to a substantially mid point of the row, and the reference current is a sum of currents flowing through the plurality of reference current generating devices. The plurality of reference current generating devices may include two devices. A first one of the two devices is disposed at a first end of the row and a second one of the two devices is disposed at a second end of the row opposite to the first end. Alternatively, the first one of the two devices may be disposed at a first location in the row and a second one of the two devices may be disposed at a second location in the row.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.